Use of an iron-base amorphous alloy having less power loss as an iron core of a transformer or a motor has heretofore been studied and has been put into practice in some transformers. However, the amorphous alloy has not been practically used in motors and is used only for wound iron cores even in transformers. This is because the sheet thicknesses of amorphous alloy foil strips produced in an industrial scale are as extremely thin as 25 μm or below. If thick foil strips are produced industrially, the amorphous alloy can be also used in motors and in stacked iron-core transformers. An increase in the thickness of foil strips improves operation efficiency of iron-core production processes and also enhances a space factor. Moreover, mechanical strength of an iron core is significantly enhanced by improving rigidity of the foil strips. In other words, the amorphous alloy can be used for a motor provided with an iron core formed by stacking the foil strips or for a stacked iron core.
The most common method for producing an amorphous alloy is a roll liquid quenching method including quenching and solidifying a molten alloy into a foil strip shape by bringing the molten alloy into contact with an outer circumferential surface of a roll, made of metal or an alloy having high thermal conductivity, while rapidly rotating the roll. However, there is a stringent restriction of the sheet thickness of the amorphous alloy foil strip that can be produced by the roll liquid quenching method and it has therefore not been possible to produce a sufficiently thick foil strip.
To address this issue, the inventors of the invention have developed a multiple-slit nozzle method using multiple slits arranged in a circumferential direction of a roll, and have disclosed the method in Patent Document 1. According to this multiple-slit nozzle method, a molten alloy ejected from the slits forms multiple puddles in a small space between the nozzle and the roll, the number of puddles corresponding to the number of the slits. A first puddle, counted from an upstream, around a contact surface thereof with the roll is cooled down on an outer circumferential surface of the roll, whereby a supercooled fluid layer with increased viscosity is drawn by the roll and a puddle on a downstream side is superimposed thereon. The temperature of the fluid layer drawn from the upstream puddle is lowered before the fluid layer meets the downstream puddle. Accordingly, the downstream puddle is cooled down by this fluid layer and a portion with increased viscosity is drawn out. A thick foil strip is formed by repeating this operation. As the fluid layers are superimposed on one another in a liquid state, interfaces thereof are mixed together so that an integrated amorphous alloy foil strip without interlayer boundaries can be obtained.
However, even the multiple-slit nozzle method has the following problem. Specifically, the roll liquid quenching method includes a method using a non-water-cooled roll and a method using a water-cooled roll. The non-water-cooled roll cools the molten alloy down by a heat capacity of the roll itself. In the case of using the non-water-cooled roll, it is possible to cool the molten alloy down efficiently and to produce a certain amount of thick amorphous alloy foil strip at an initial producing state when the roll temperature is low. However, the non-water-cooled roll reduces cooling efficiency when the roll temperature is increased and therefore cannot be used for a long period of time. Accordingly, this is not suitable to produce the amorphous alloy foil strips industrially.
Due to this reason, it is preferable to use the water-cooled roll from an industrial perspective. As a water cooling mechanism is embedded in the water-cooled roll, it is possible to radiate the heat by way of cooling water even when the roll itself has a small heat capacity. However, the thick amorphous alloy having a sheet thickness exceeding 25 μm has been difficult to mass-produce in an industrial scale even by using the water-cooled roll.    [Patent Document 1] JP S60-108144 A    [Patent Document 2] JP H6-86847 U    [Patent Document 3] JP S61-059817 B